Has anybody actually placed a REAL horizontal antenna close to salt water,
then placed an identical REAL antenna back several wavelengths and A-B'd
the difference over various times and propagation conditions? If so, what
differences, if any, were observed?
73
tom K4RV
> [Original Message]
> From: Jim Lux <jimlux@earthlink.net>
> To: <towertalk@contesting.com>; Stephen Reichlyn
<Stephen.Reichlyn@ryansci.com>
> Date: 1/18/2005 16:16:27
> Subject: Re: [TowerTalk] horizontally polarized antennas and salt water
>
> I'll take a shot at some "qualitative" answers, based entirely on
> theory...Others will pitch in with practical experience, which ALWAYS
varies
> from theory.
>
> ----- Original Message -----
> From: "Stephen Reichlyn" <Stephen.Reichlyn@ryansci.com>
> >
> > We have all read that vertical antennas work especially well on (or
near)
> > the beach. I have first hand experience
> > with this and certainly agree that this is the case. I often wondered
> > whether radials (even simple 2 wire systems)
> > are really needed next to salt water.
> >
> > 1.The ultimate question is 'what effect does the salt water have on
> > horizontally polarized HF antennas, if qny'?
>
> Not nearly as much as for verticals. For horizontally polarized waves,
the
> reflection coefficient is pretty high for almost any incidence angle,
> regardless of soil properties. For vertically polarized waves, though,
this
> isn't the case.
>
> >
> > 2. Do you think that tidal changes (for instance +/- 6 feet) in the near
> > field have an effect on angle of radiation
> > of horizontally polarized antennas, such as yagis.
>
> Kind of depends on what you mean by "near field"...
>
> The effect would be of the same nature as raising or lowering the antenna
by
> that amount... for a moderately high antenna, that may not be all that
> significant (if even measurable)..
>
> There would also be an effect from resistive losses in the near field
> changing.
>
> >
> > 3. How far away from the ocean (in miles) do you think the near field
salt
> > water effect disappears?
>
> The near field ends (by convention) where the amount of energy radiated
away
> (per unit area) is equal to the amount of energy kept in the antenna).. By
> convention (especially for simple antennas) the boundary is at
> 2*pi*wavelengths away...For high gain antennas with lots of coupling among
> elements, this may not hold.
>
> By the way, the polarization of the E or H field in the near field at a
> given point doesn't necessarily correspond to the far field polarization.
> You might have an antenna with currents in vertical members where they
> cancel in the far field, but in the near field, they aren't. Think of two
> vertically stacked horizontal 1/2 wavelength dipoles connected at the ends
> by 1 wavelength long wires, with only the bottom element fed. Close to
one
> or the other end, there's a significant vertical component, but in the far
> field, the vertical components from the two ends will cancel.
>
>
> The pattern effects (from reflection coefficient and polarization) will be
> affected many wavelengths away, particularly at low angles. For instance,
> at 6 degrees (about 1/10 radian), the "spot" where the ray is reflected is
> about 10 antenna heights away. (tan(6 deg) = 0.1). These are (in antenna
> analysis) far field effects, because the reflection coefficient at that
> point does not affect the current distribution in the antenna, and are
more
> properly described as "propagation effects".
>
> But hey, in practice, they're both important in antenna siting (that's why
> it's the IEEE Transactions on Antennas AND Propagation)
>
> >
> > 4. Or do you think that salt water near field has NO effect on
> horizontally
> > polarized antennas?
> >
> > 73,
> > Steve AA4V
> >
> > Stephen Reichlyn
> > Ryan Scientific, Inc.
> > CALL toll-free in North America
> > 888-884-4911
> > 843-884-4911 ext 302
> > 843-884-5568 (fax)
> > www.ryansci.com
> > _______________________________________________
> >
> > See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
> Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
any
> questions and ask for Sherman, W2FLA.
> >
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>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
> _______________________________________________
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_______________________________________________
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Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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